Monitor cameras and in-vehicle cameras, including a combination of imaging lenses and area sensors, are being put into practical use.
The imaging lens, which is used in monitor cameras or in-vehicle cameras, preferably has a wide imaging range. Therefore, the imaging lens needs to have a wide field angle.
The imaging lens is also preferably high-resolution.
For example, when the imaging lens is used in an in-vehicle camera, the imaging lens is required to have sufficiently high resolution for the following purpose. That is, the imaging lens is used for observing road conditions at close range (to observe whether there are any small dangerous articles such as nails and glass fragments), and for observing distant road conditions on a highway where the vehicle is travelling at high speed.
In another example, when the imaging lens is used in a monitor camera for security, the imaging lens is required to have sufficiently high resolution for the following purposes. Specifically, the imaging lens is used for clearly imaging/observing a person and belongings of the person (dangerous articles such as a knife).
Furthermore, monitor cameras and in-vehicle cameras are often used outdoors. Thus, the brightness of the environment changes significantly between daytime and nighttime. In order to successfully perform imaging at nighttime, the imaging lens needs to have high brightness.
Furthermore, there is demand for compact monitor cameras and compact in-vehicle cameras. Therefore, it is important that the size of the lens is compact.
Conventionally, there is an imaging lens including seven lenses that may be used in monitor cameras or in-vehicle cameras (see patent documents 1 and 2). This imaging lens has a wide angle, relatively good performance, high brightness, and a relatively compact size.
The imaging lenses disclosed in patent documents 1 and 2 both have a wide angle exceeding 180 degrees. Furthermore, they both include a small number of lenses (seven lenses) and are thus advantageously compact in size.
However, in order to increase the resolution of an imaging lens having a sufficiently wide angle for in-vehicle cameras and monitor cameras, the chromatic aberration needs to be corrected properly.
In the imaging lens described in patent document 1, the chromatic aberration between the red light and the blue light is approximately 0.05 mm (50 μm). Thus, when an imaging element having a pixel pitch of approximately 6 μm is used, the imaging positions of the red light and the blue light are displaced by eight pixels or more in the same image. Consequently, the formed color image will have low resolution.
As to the imaging lens disclosed in patent document 2, the extent to which the chromatic aberration is corrected is unknown, and therefore the resolution is unknown.
Patent Document 1: Japanese Laid-Open Patent Application No. 2006-337691
Patent Document 2: Japanese Laid-Open Patent Application No. 2005-345577
Patent Document 3: Japanese Laid-Open Patent Application No. 2008-276185